The present invention relates to a process and equipment used in the forming of a paper web. More particularly, the invention relates to a process in the forming of paper web and in the dewatering of the pulp web and of the paper web formed.
The invention further relates to a twin wire former intended for carrying out the process of the invention. The former comprises a loop of a carrying wire guided by the breast roller, the forming roller and the guide rollers, as well as a loop of a covering wire guided by the breast roller, the forming roller and the guide rollers. The wire loops together form a forming gap between and in connection with the breast rollers. The pulp suspension jet is supposed to be fed into the forming gap. The forming gap is followed by a joint twin-wire forming and dewatering zone of the wires. The web is arranged after the zone, so as to follow along with the carrying wire, from which the web is detached and passed into the drying section of the paper machine.
As the running speeds of paper machines are increased, several problems in the forming of the web are accentuated even further. Phenomena that act in the forming section of a paper machine upon the fiber mesh and upon the water that is still relatively free in connection with said mesh, in particular the force effects, are usually intensified in proportion to the second power of the web speed. The maximum web speeds of the present newsprint machines are of the order of 1200 meters per minute. Newsprint machines are, however, being planned in which a web speed of up to about 1500 m/min is aimed at. Such increase in speed causes several problems, which will be discussed in the following.
A so-called hybrid former is a former in which the forming zone has a single-wire initial portion, onto which the headbox feeds the pulp suspension jet. A twin wire forming zone follows the single-wire portion. A problem of hybrid formers, as of four-drinier formers, is that at high web speeds splashes occur in the pulp web. These splashes result from the collision angle between the pulp jet and the forming board and, on the other hand, from the scattering of the highly turbulent pulp jet as said jet meets the forming board. The reach of the splashes in the direction of the pulp web is quite long, and these splashes cause marks in the pulp web being formed and thereby deteriorates the quality of the paper produced. On the other hand, the foil pulses used for the removal of water from a fourdrinier former become so high at high speeds that this causes splashing which deteriorates the formation of the web. As is well known, the foil pulsation increases proportionally to the second power of the speed. In order that the pulsation be maintained below the splashing limit at a high speed, the foil angles must be made so small (approaching the angle 0.degree.) that an adequate dewatering capacity is not obtained.
It is a further drawback of a fourdrinier former that transverse profile defects present in the discharge jet may be accentuated further on the fourdrinier wire, for example, due to diagonal flow components in the pulp slurry (so-called plowings on the wire board), or in the form of stronger longitudinal streaks.
It is a common opinion that the variations in grammage in twin gap formers remain lower than in fourdrinier formers or hybrid formers. This is due to the fact that in gap formers, the jet is supplied straight into the gap, wherein the pulp jet is immediately "supported" between two wires, so that no transverse flows can arise, which transverse flows would intensify the defects in profile.
When the speeds of paper machines, in particular of newsprint machines, increase, uniformity of the web is, besides being a factor of paper quality, also important, since uniformity of the web has an ever higher effect on the running quality of the paper machine, because the weakest portions of the web are, as a rule, the cause of the breaks.